1. Field of the Invention
The present invention relates to an electron emitting device and a driving method thereof.
2. Description of the Related Art
As mechanisms which emit electrons from an interior of a solid to an outside, a thermoelectronic emission, a photoelectron emission, a field electron emission, a secondary electron emission, and the like have been typically known.
Electron emitting devices having a field electron emission mechanism in these mechanisms, that is, an MIM (metal insulator metal) electron emitting device and an MIS (metal insulator semiconductor) electron emitting device have been known. These electron emitting devices are electron emitting devices of the surface-emitting type each of which uses a quantum size effect and an intense electric field in an interior of the electron emitting device to accelerate electrons and emits the electrons from its plate-shaped device surface. Since the electrons accelerated in electron acceleration layers in the interior of the devices are emitted to an outside, these electron emitting devices do not require an intense electric field outside the devices.
On the contrary, in a Spindt type electron emitting device and a CNT type electron emitting device, which use an intense electric field outside the devices, the device itself is likely to be broken down due to a sputtering with an ionization of gaseous molecules, so that a handling in low vacuum is troublesome. Therefore, attention has been given to the MIM electron emitting device and the MIS electron emitting device, which can emit electrons, not only in low pressure, but also in atmospheric pressures, thereby advancing their technical development.
For example, it has been reported that an electron source called a ballistic electron surface-emitting device (BSD) has an excellent characteristic in low vacuum (see J. Vac. Sci. Technol. B 23 2336-2339 (2005). “Operation of nanocrystalline silicon ballistic emitter in low vacuum and atmospheric pressures.”).
In addition, an electron emitting device which improves a stability in atmospheric pressures as compared with the conventional MIM electron emitting device and the MIS electron emitting device has been developed. For example, an electron emitting device has been known in which an electron acceleration layer including conductive particles which are configured of electric conductors and have a strong anti-oxidant action and insulating substances larger than the conductive particles is provided between a substrate having a lower electrode and an upper electrode configured of a conductor thin film. It has been known that the electron emitting device can stably emit electrons in atmospheric pressures (see, Japanese Unexamined Patent Publication No. 2009-146891).
In the ballistic electron surface-emitting device, nanometer order silicon microcrystals and a silicon oxide film covering the microcrystals continuously and alternately provide an electron transit space which is not subjected to an electron scattering and an electron acceleration field of a local intense electric field, thereby emitting electrons.
However, when the electron source is driven in atmospheric pressures, in particular, in a room atmosphere including oxygen and water vapor, the silicon microcrystals combine with the oxygen in the atmosphere, with the result that the entire microcrystals are modified to an oxidized silicon. Therefore, the structure itself which generates ballistic electrons is lost.
Therefore, it is difficult to drive the electron source for a long period of time of several hours or several hundred hours in atmospheric pressures, in particular, in a room atmosphere including oxygen, so that a device which can be driven for a long period of time in the atmosphere has been desired.
On the other hand, the electron emitting device in which the electron acceleration layer includes conductive particles and insulating substances is configured of a material which is hard to be oxidized as compared with the ballistic electron surface-emitting device, thereby having an excellent characteristic in atmospheric pressures (room atmosphere).
However, in such a electron emitting device, since the electron acceleration layer includes the insulating substances as a main component, electrons are likely to be captured into the electron acceleration layer (an electron charging is likely to occur). When the electrons are captured, the captured electrons cause a local release of an acceleration electric field in the electron acceleration layer to make an acceleration of the electrons insufficient, thereby deteriorating an electron emission of the electron emitting device. The electron capture occurs continuously when an acceleration voltage is applied, with the result that when a direct current voltage is applied continuously, an electron emission amount is lowered with an elapse of voltage application time (driving time) although the device is not physically broken down. Such a lowered electron emission amount can be coped with by increasing the acceleration voltage, but when the electron capture occurs again, the electron emission amount is lowered eventually (therefore, even when the acceleration voltage is increased until the electron acceleration layer is electrically broken down, a stable electron emission amount cannot be maintained).
In this way, it is difficult for the electron emitting device to stably emit electrons for a long period of time in atmospheric pressures, in particular, in a room atmosphere including oxygen, so that a device which can stably emit electrons for a long period of time in an atmosphere has been desired.
In the electron emitting device, when a direct current voltage is applied continuously in atmospheric pressures (in particular, in a room atmosphere including water vapor), an electromigration of a substrate metal can occur to physically break the electron acceleration layer down. Therefore, a device which can be driven for a long period of time in an atmosphere has been desired.